The term diesel-electric locomotive designates a locomotive the propulsion of which is ensured by several electric motors, the electrical energy powering the motors being provided by a diesel engine supplied with heavy fuel oil and driving an alternator.
In such locomotives, the alternator powers, through a rectifier, a direct current bus to which the different electric traction motors of the locomotive are connected via inverters.
The locomotive also includes auxiliary equipment, such as cooling fans or compressors that, depending on the case, are either directly mechanically connected to the shaft of the diesel engine, or powered by an alternator specific to them, said alternator being driven by the diesel engine.
In both cases, the operating speed of the auxiliary equipment is directly connected to the speed of rotation of the diesel engine.
As a result, to allow a satisfactory operation of this auxiliary equipment during electric braking operating phases of the locomotive or stopping thereof, it is necessary to keep the diesel engine at a high enough rating. This leads to significant heavy fuel oil consumption to power the locomotive.